This invention relates to a multicomponent copolymer which uses a fluoroolefin and a carboxylic acid vinyl ester as the basic components and further incorporates a hydroxyl-containing allyl ether and an unsaturated carboxylic acid in order to possess both hydroxyl group and carboxyl group. The copolymer is soluble in various organic solvents, curable by using the hydroxyl group as a functional group and suitable for use in paints including electrodepositon paints.
Fluororesins are generally excellent in chemical resistance, weather resistance and heat resistance, and in view of these favorable properties the use of fluororesin based paints is increasing in various fields.
As a fluororesin suitable for solvent-thinned paints, U.S. Pat. No. 4,631,326 shows a copolymer of chlorotrifluoroethylene with a carboxylic acid vinyl ester such as vinyl acetate and a hydroxyl-containing allyl ether such as ethylene glycol monoallyl ether. This copolymer is soluble in various organic solvents and curable with a conventional curing agent which reacts with hydroxyl group. The patent specification contains an example of preparing this copolymer by a suspension polymerization method, but usually the copolymer is prepared by a solution polymerization method. U.S. Pat. No. 4,954,589 relates to an analogous copolymer and shows to introduce carboxyl group into the copolymer by using an acid such as acrylic acid or vinylacetic acid as an additional monomer with the purpose of improving the dispersibility of inorganic and organic pigments in a solution of the copolymer. Usually the carboxyl-containing copolymer is prepared by solution polymerization.
In preparing a copolymer for use in solvent-thinned paints it is favorable to employ a solution polymerization method to thereby obtain the copolymer in the form of solution. A problem inherent to solution polymerization is that some portions of the monomers charged in the reactor inevitably remain unreacted and exist in the obtained solution of the copolymer. The unreacted residual monomers include monomers having a functional group such as hydroxyl group or carboxylic group, and when the copolymer solution is used in a paint composition the properties of the paint film are adversely affected by the reaction of the functional groups of the residual monomers with a curing agent.
Recently the application of electrodeposition painting is widening, and it has been developed to use fluororesins that are soluble in organic solvents in electrodeposition paints for vehicles or other articles to be used outdoors. For use in electrodeposition painting there is the need of preparing an acidified fluororesin. For example, U.S. Pat. No. 4,711,926 and U.S. Pat. No. 4,842,940 respectively show electrodeposition paints using fluoroolefin based copolymers having both hydroxyl group and carboxyl group. Either of these copolymers is prepared by a two-stage process: by first forming a copolymer having hydroxyl group but not having carboxyl group by solution polymerization and then converting a portion of the hydroxyl groups in the copolymer to carboxyl groups.
In electrodeposition painting, it is very undesirable that a resin used in the paint contains unreacted monomers having functional groups because such residual monomers are detrimental to the stability of the electrodeposition bath and adversely affect the curability and adhesibility of the paint film. To reduce the contents of unreacted residual monomers in a copolymer prepared by solution polymerization, the conventional method is distillation under reduced pressure. However, the monomers having hydroxyl group or carboxyl group having relatively high boiling points and hence cannot easily be decreased by distillation. Besides, to use a copolymer prepared by a solution polymerization method in an electrodeposition paint there is the need of completely distilling out the organic solvent used in the solution polymerization operation and then dissolving the dried copolymer in another organic solvent which is soluble in water.